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Chapter 26
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Nutrition and
Metabolism
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Introduction
• Nutrition is the starting point and the basis for all
human form and function
– The source of fuel that provides the energy for all
biological work
– The source of raw materials for replacement of wornout biomolecules and cells
• Metabolism is the chemical change that lies at the
foundation of form and function
26-2
Body Weight and Energy Balance
• Weight—determined by the body’s energy balance
– If energy intake and output are equal, body weight is
stable
– Gain weight if intake exceeds output
– Lose weight if output exceeds intake
– Seems to have a stable, homeostatic set point
• Varies from person to person
• Combination of heredity and environmental influences
– 30% to 50% hereditary
– Rest environmental factors such as eating and exercise habits
26-3
Appetite
• Control of appetite and body weight includes a
still-growing list of peptide hormones and
regulatory pathways that control short- and longterm appetite
– Gut–brain peptides: act as chemical signals from the
gastrointestinal tract to the brain
• Short-term regulators of appetite
– Mechanisms work over periods of minutes to hours
– Makes one feel hungry and begin eating
– Makes one feel satiated and end a meal
26-4
Appetite
• Short-term regulator—effects last minutes to hours
– Ghrelin
• Secreted from parietal cells in fundus of empty stomach
• Produces sensation of hunger
• Stimulates the hypothalamus to secrete growth hormone–
releasing hormone
– Primes the body to take best advantage of the nutrients about
to be absorbed
• Ghrelin secretion ceases within an hour of eating
• Signal that begins a meal
26-5
Appetite
Cont.
– Peptide YY (PYY)
• Secreted by enteroendocrine cells of ileum and colon
• Sense that food has arrived in the stomach
• Secrete PYY long before chyme reaches the ileum in
amounts proportionate to calories consumed
• Primary effect is to signal satiety and terminate eating
• Signal that ends a meal
– Cholecystokinin (CCK)
• Secreted by enteroendocrine cells in duodenum and
jejunum
• Stimulates the secretion of bile and pancreatic enzymes
• Stimulates the brain and sensory fibers of the vagus nerve
suppressing appetite
26-6
Appetite
• Long-term regulators—governs one’s average
rate of caloric intake and energy expenditure over
periods of weeks to years
• These two peptides inform the brain of how much
adipose tissue the body has and activates mechanisms
for adding or reducing fat
– Leptin
•
•
•
•
Secreted by adipocytes throughout the body
Level proportionate to one’s own fat stores
Informs brain on how much body fat we have
Obese people are more likely to have a receptor defect
than hormone deficiency
26-7
Appetite
Cont.
– Insulin
•
•
•
•
Secreted by pancreatic beta cells
Stimulates glucose and amino acid uptake
Promotes glycogen and fat synthesis
Has receptors in the brain and functions, like leptin, as an
index of the body’s fat stores
• Weaker effect on appetite than leptin
26-8
Gut–Brain Peptides in Appetite Regulation
Satiety
Hunger
Forebrain
–
+
NPYsecreting
neurons
Arcuate nucleus
of hypothalamus
Melanocortinsecreting
neurons
+
–
Key
Secretion
+ Stimulatory
effect
–
Inhibitory
effect
Insulin
Ghrelin
Leptin
PYY
CCK
Figure 26.1
PYY
Small
intestine
Pancreas
Stomach
Adipose tissue
Large
intestine
PYY
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
26-9
Obesity
• Obesity—weight more than 20% above
recommended norm for one’s age, sex, and height
– U.S. rates
• 30% obese
• 35% overweight
• Body mass index (BMI)—indication of overweight
or obese
– BMI = W/H2 (W = weight in kg; H = height in meters)
• 20 to 25 is optimal for most people
• Over 27: overweight
• Above 30: obese
26-10
Obesity
• Shortens life expectancy
– Increased risk of atherosclerosis; hypertension;
diabetes mellitus; joint pain; kidney stones and
gallstones; cancer of uterus, breast, and prostate; and
sleep apnea
• Causes are diverse and unknown
– Heredity, overfeeding in infancy, and problems with
appetite and weight-regulating mechanisms are
suspected
26-11
Appetite
• Hunger is stimulated partly by gastric peristalsis
– Mild hunger contractions begin soon after stomach is
empty
– Increase in intensity over a period of hours
– Do not affect the amount of food consumed
26-12
Appetite
• Appetite is briefly satisfied by:
– Chewing and swallowing
– Stomach filling
– Lasting satiation depends upon nutrients entering blood
• Neurotransmitters stimulate desire for different
types of food
– Norepinephrine: carbohydrates
– Galanin: fats
– Endorphins: protein
26-13
Calories
• One calorie—amount of heat required to raise
temperature of 1 g of water 1°C
– 1,000 calories is a kilocalorie (kcal) or Calorie
– A measure of the capacity to do biological work
• Carbohydrates and proteins yield about 4
kcal/g
– Sugar and alcohol (7.1 kcal/g) are “empty” calories
• Few nutrients and suppress appetite
26-14
Calories
• Fats yield about 9 kcal/g
• Good nutrition requires complex foods that meet
the body’s needs for protein, lipid, vitamins, and
other nutrients
• Fuel—substance solely or primarily oxidized to
extract energy from it
– Extracted energy used to make adenosine
triphosphate (ATP)
26-15
Nutrients
• Nutrient—any ingested chemical used for growth,
repair, or maintenance of the body
• Six classes of nutrients
– Water, carbohydrates, lipids, and proteins
• Macronutrients—must be consumed in relatively large
quantities
– Vitamins and minerals
• Micronutrients—only small quantities are required
26-16
Nutrients
• Recommended daily allowances (RDA)
– Safe estimate of daily intake that would meet the
nutritional needs of most healthy people
• Essential nutrients cannot be synthesized in body
– Minerals, most vitamins, eight amino acids, and one to
three of the fatty acids must be consumed in diet
26-17
Carbohydrates
• Well-nourished adult body has 440 g of
carbohydrates
– 325 g of muscle glycogen
– 90 to 100 g of liver glycogen
– 15 to 20 g of blood glucose
• Sugars function as:
– Structural components of other molecules including
nucleic acids, glycoproteins, glycolipids, ATP, and
related nucleotides (GTP, cAMP)
26-18
Carbohydrates
• Blood glucose concentration carefully regulated
– Interplay of insulin and glucagon
– Regulate balance between glycogen and free glucose
• Carbohydrate intake influences metabolism of other
nutrients
– Fats used as fuel when glucose and glycogen levels are
low
– Excess carbohydrates are converted to fat
26-19
Carbohydrates
• Dietary carbohydrates in three principal forms
– Monosaccharides: glucose, galactose, fructose
• Arise from digestion of starch and disaccharides
• Small intestine and liver convert galactose and fructose to
glucose
– Ultimately, all carbohydrate digestion generates glucose
– Outside hepatic portal system, only blood sugar is glucose
– Normal blood sugar concentration: 70 to 110 mg/dL
– Disaccharides: sucrose (table sugar), maltose, lactose
– Polysaccharides (complex carbohydrates): starch,
glycogen, and cellulose (not a nutrient because it is not
digested, but important as dietary fiber)
26-20
Fiber
• Dietary fiber—all fibrous material of plant and
animal origin that resists digestion
– Cellulose, pectin, gums, and lignins
• Fiber is important to diet—RDA is 30 g/day
• Water-soluble fiber (pectin)
– Found in oats, beans, peas, brown rice, and fruits
– Decreases blood cholesterol and LDL levels
26-21
Fiber
• Water-insoluble fiber (cellulose, hemicellulose,
lignin)
– No effect on cholesterol and LDL levels
– Absorbs water in intestines, softens stool, increases bulk
40% to 100%, stretches colon, and stimulates peristalsis
thereby quickening passage of feces
– No clear effect on incidence of colorectal cancer
– Excessive intake can interfere with absorption of
elements such as iron, calcium, magnesium,
phosphorus, and other trace elements
26-22
Lipids
• Reference male 15% body fat; reference female
25% body fat
• Well-nourished adult meets 80% to 90% of resting
energy needs from fat
– Fat is superior to carbohydrates for energy storage for
two reasons
• Carbohydrates are hydrophilic, absorb water, and expand
and occupy more space in the tissues, and fat is
hydrophobic, contains almost no water, and is a more
compact energy storage substance
• Fat is less oxidized than carbohydrates and contains over
twice as much energy: 9 kcal/g for fat; 4 kcal/g for
carbohydrates
26-23
Lipids
• Fat has glucose-sparing and protein-sparing
effects when used for energy needs
– Glucose is spared for consumption by cells that cannot
use fat, like neurons
– Protein not catabolized for fuel
• Fat-soluble vitamins (A, D, E, K) absorbed with
dietary fat
– Ingestion of less than 20 g/day risks vitamin deficiency
26-24
Lipids
• Diverse functions besides energy source
– Structural
• Phospholipids and cholesterol are components of
plasma membranes and myelin
– Chemical precursors
• Cholesterol—a precursor of steroids, bile salts, vitamin D
• Thromboplastin, an essential blood-clotting factor, is a
lipoprotein
• Fatty acids—arachidonic acid and linoleic acid:
precursors of prostaglandins and other eicosanoids
– Important protective and insulating functions
26-25
Lipids
• Requirements: should be less than 30% of daily
calorie intake
– Typical American gets 40% to 50% from fat
• Most fatty acids synthesized by body
– Essential fatty acids must be consumed
26-26
Lipids
• Sources
– Saturated fats
• Animal origin—meat, egg yolks, dairy products
– Unsaturated fats
• Found in nuts, seeds, and most vegetable oils
– Cholesterol
• Found in egg yolks, cream, shellfish, organ meats, and
other meats
• Does not occur in foods of plant origin
26-27
Cholesterol and Serum Lipoproteins
• Serum lipoproteins are classified into four major
categories by their density
–
–
–
–
Chylomicrons: 75–1,200 nm in diameter
Very low–density lipoproteins (VLDLs): 30–80 nm
Low-density lipoproteins (LDL): 18–25 nm
High-density lipoproteins (HDL): 5–12 nm
• Their most important differences are in composition
and function
26-28
Lipoprotein Processing
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Chylomicron
Very low–density
lipoprotein
Low-density
lipoprotein (LDL)
High-density
lipoprotein (HDL)
Key
Phospholipid
Phospholipid (3%)
Phospholipid (17%)
Phospholipid (21%)
Phospholipid (25%)
Triglyceride
Triglyceride (90%)
Triglyceride (55%)
Triglyceride (6%)
Triglyceride (5%)
Cholesterol
Cholesterol (5%)
Cholesterol (20%)
Cholesterol (53%)
Cholesterol (20%)
Protein
Protein (2%)
Protein (8%)
Protein (20%)
Protein (50%)
(a) Lipoprotein types
Figure 26.2a
26-29
Cholesterol and Serum Lipoproteins
• Chylomicrons form in absorptive cells of small
intestine
• Enter lymphatic system, then bloodstream
• Blood capillary endothelial cells have lipoprotein
lipase to hydrolyze triglycerides into
monoglycerides and free fatty acids
26-30
Cholesterol and Serum Lipoproteins
• Most of the body’s cholesterol is endogenous—
internally synthesized rather than dietary
–
–
–
–
Body compensates for variation in intake
High dietary intake lowers liver cholesterol production
Low dietary intake raises liver production
Lowering dietary cholesterol lowers level by no more
than 5%
– Certain saturated fatty acids (SFAs) raise serum
cholesterol level
• Moderate reduction in SFAs can lower blood cholesterol
by 15% to 20%
26-31
Cholesterol and Serum Lipoproteins
• Vigorous exercise lowers blood cholesterol
– Sensitivity of right atrium to blood pressure is reduced
– Heart secretes less atrial natriuretic peptide and thus
kidneys excrete less sodium and water
– Raises blood volume
– Dilution of blood lipoproteins causes adipocytes to
produce more lipoprotein lipase
– Adipocytes consume more blood triglycerides
– VLDL particles shed some cholesterol which is picked
up by HDL and removed by the liver
26-32
Cholesterol and Serum Lipoproteins
• Levels of LDL
– High LDL is a warning sign
– Correlates with cholesterol deposition in arteries
– Elevated by saturated fat intake, cigarette smoking,
coffee, and stress
• Levels of HDL
– High level of HDL is beneficial
– Indicates that cholesterol is being removed from the
arteries and transported to the liver for disposal
26-33
Cholesterol and Serum Lipoproteins
• Recommendations
– Increase your ratio of HDL to LDL
– Regular aerobic exercise
– Avoid smoking, saturated fats, coffee, stress
26-34
Lipoprotein Processing
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Chylomicron
pathway
Lymph absorbs
chylomicrons
from small intestine
Cells absorb LDLs
by receptor-mediated
endocytosis
VLDL/LDL
pathway
Lymph drains
into bloodstream
Triglycerides
removed and stored
in adipocytes
Lipoprotein lipase removes
lipids from chylomicrons
Liver disposes
of chylomicron
remnants
Lipids are stored in
adipocytes or used
by other cells
VLDLs become LDLs
containing mainly
cholesterol
Liver
produces
VLDLs
Liver produces
empty HDL shells
HDL shells pick
up cholesterol and
phospholipids
from tissues
Filled HDLs
Liver excretes
return to liver
excess cholesterol
and bile acids
HDL
pathway
(b) Lipoprotein-processing pathways
Figure 26.2b
26-35
Proteins
• Protein constitutes 12% to 15% of total body mass
– 65% of it is in skeletal muscle
• Functions
– Muscle contraction
– Motility of cilia and flagella
– Structural components
• All cellular membranes
– Receptors, pumps, ion channels, and cell-identity markers
• Fibrous proteins
– Collagen, elastin, and keratin make up much of the structure
of bone, cartilage, tendons, ligaments, skin, hair, and nails
26-36
Proteins
Cont.
• Globular proteins
– Antibodies, hormones, myoglobin, neuromodulators,
hemoglobin, and about 2,000 enzymes that control nearly
every aspect of cellular metabolism
• Plasma proteins
– Albumins and other plasma proteins that maintain blood
viscosity and osmolarity and transport lipids and some other
plasma solutes
– Buffer pH of body fluids
– Contributes to resting membrane potential of all
cells
26-37
Proteins
• Protein RDA is 44 to 60 g/day
– Weight in pounds x 0.37 = estimate of RDA of protein
– Higher intake recommended under conditions of stress,
infection, injury, and pregnancy
– Excessive intake overloads the kidneys with nitrogenous
waste and can cause kidney damage
26-38
Proteins
• Nutritional value of a protein depends on right
proportions of amino acids needed for human
proteins
– 8 essential amino acids cannot be synthesized by the
body
• Isoleucine, leucine, lysine, methionine, phenylalanine,
threonine, tryptophan, and valine
– 12 inessential amino acids synthesized by the body if
the diet does not supply them
• Cells do not store surplus amino acids for later use
– When a protein is synthesized, all amino acids must be
present at once
– If one is missing, the protein cannot be synthesized
26-39
Proteins
• Complete proteins—high-quality dietary proteins
that provide all of the essential amino acids in the
necessary proportions for human tissue growth,
maintenance, and nitrogen balance
• Incomplete proteins—lower quality because they
lack one or more essential amino acids
26-40
Minerals and Vitamins
• Minerals—inorganic elements that plants extract
from the soil or water and introduce into the food
web
• Vitamins—small dietary organic compounds that
are necessary to metabolism
• Neither is used as fuel
• Both are essential to our ability to use other
nutrients
26-41
Minerals and Vitamins
• Minerals constitute about 4% of the body mass
– Three-quarters being calcium and phosphorus in
bones and teeth
– Phosphorus
• Key structural component of phospholipids, ATP, cAMP,
GTP, and creatine phosphate
• Basis for the phosphate buffer system
– Calcium, iron, magnesium, and manganese function
as cofactors for enzymes
26-42
Minerals and Vitamins
Cont.
– Iron is essential for the oxygen-carrying capacity of
hemoglobin and myoglobin
– Chlorine: component of stomach acid
– Many mineral salts function as electrolytes and govern
function of nerve and muscle cells, osmotically regulate
the content and distribution of water in the body, and
maintain blood volume
• Plants are the best source of minerals
26-43
Minerals
• Vegetables, legumes, milk, eggs, fish, and shellfish
• Animal tissues contain large amounts of salt
– Carnivores rarely lack salt in their diets
– Herbivores often supplement by ingesting salt from soil
• Recommended sodium intake is 1.1 g/day
• Typical American diet contains 4.5 g/day
• Hypertension can be caused by elevated salt
intake
26-44
Vitamins
• Water-soluble vitamins (C, B complex)
– Absorbed with water in small intestine
– Quickly excreted by the kidneys, not stored, and
seldom accumulate to excess
26-45
Vitamins
Cont.
– Vitamin C (ascorbic acid)
• Promotes hemoglobin synthesis, collagen synthesis, and
sound connective tissue structure
• An antioxidant that scavenges free radicals and possibly
reduces the risk of cancer
– B vitamins
• Function as coenzymes or parts of coenzyme molecules
• Assist enzymes by transferring electrons from one
metabolic reaction to another
• Making it possible for enzymes to catalyze these reactions
26-46
Vitamins
• Fat-soluble vitamins
– Incorporated into lipid micelles in the small intestine and
absorbed with dietary lipids
– Vitamin A
• Component of visual pigments
• Promotes proteoglycan synthesis and epithelial
maintenance
– Vitamin D
• Promotes calcium absorption and bone mineralization
– Vitamin K
• Essential for prothrombin synthesis and blood clotting
– Vitamins A and E
• Antioxidants like ascorbic acid
26-47
Vitamins
• Diseases resulting from vitamin deficiencies or
excesses
• Hypervitaminosis—vitamin excess
• Vitamin A deficiency—night blindness, dry skin
and hair, dry conjunctiva and cloudy cornea, and
increased incidence of urinary, digestive, and
respiratory infections
– World’s most common vitamin deficiency
26-48
Vitamins
• Vitamin A excess—may cause anorexia, nausea
and vomiting, headache, pain and fragility in the
bones, hair loss, an enlarged liver and spleen, and
birth defects
• Vitamins B6, C, D, and E have also been
implicated in toxic hypervitaminosis
• Megavitamins—doses 10 to 1,000 times the RDA
show no effect on performance
26-49
Carbohydrate Metabolism
• Dietary carbohydrate burned as fuel within hours of
absorption
• All oxidative carbohydrate consumption is essentially a
matter of glucose catabolism
C6H12O6 + 6 O2  6 CO2 + 6 H2O
• Function of this reaction is to transfer energy from
glucose to ATP
– Not to produce carbon dioxide and water
26-50
Glucose Catabolism
• Glucose catabolism—a series of small steps,
each controlled by a separate enzyme, in which
energy is released in small manageable amounts,
and as much as possible, is transferred to ATP and
the rest is released as heat
26-51
Glycolysis and Anaerobic Fermentation
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Key
Glucose
Carbon atoms
ATP
Phosphate
groups
1 Phosphorylation
ADP
Glucose 6-phosphate
Glycogen
Fat
Fructose 6-phosphate
ATP
2 Priming
ADP
Fructose 1,6-diphosphate
3 Cleavage
2 PGAL
2 Pi
2 NAD+
2 NADH + 2 H+
4 Oxidation
2
2 ADP
2 H2O
2 ATP
2
5 Dephosphorylation
2 ADP
2 ATP
2
2 pyruvic acid
Figure 26.3
2 NADH + 2 H+
2 NAD+
2
2 lactic acid
26-52
Anaerobic fermentation
Aerobic respiration
Anaerobic Fermentation
• Anaerobic fermentation—in absence of oxygen,
the cell resorts to this one-step reaction
– NADH donates a pair of electrons to pyruvic acid
reducing it to lactic acid
– Regenerating NAD+
• Lactic acid leaves the cells that generate it
– Enter bloodstream and transported to the liver
– When oxygen becomes available the liver oxidized it
back to pyruvic acid
– Oxygen is part of the oxygen debt created by exercising
muscle
26-53
The Mitochondrial Matrix Reactions
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Pyruvic acid (C3)
6
CO2
NAD+
7
NADH + H+
Acetyl group (C2)
8
Acetyl-CoA
Coenzyme A
H2O
9
Citric acid (C6)
Oxaloacetic acid (C4)
H2O
10
NADH + H+
NAD+
(C6)
Citric
acid
cycle
18
H2O
NAD+
11
NADH + H+
(C4)
12
CO2
17
(C5)
H2O
13
Occurs in
mitochondrial
matrix
(C4)
NADH + H+
14
16
FADH2
NAD+
(C4)
Figure 26.4
CO2
FAD
(C4)
Pi
15
GTP
GDP
26-54
ADP
ATP
Electron Transport
• Oxygen is the final electron acceptor
– Each oxygen atom accepts two electrons from
cytochrome a3 and two protons from the mitochondrial
matrix forming water
• Body’s primary source of metabolic water—water
synthesized in the body
– This reaction explains the body’s oxygen requirement
– No oxygen, cell produces too little ATP to sustain life
26-55
Chemiosmotic Mechanisms of ATP Synthesis
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Intermembrane
space
Matrix
Cristae
Figure 26.6
Inner membrane
Outer membrane
NADH +
H+
½ O2 + 2 H+
NAD+
H2O
6 H+
Matrix
2e–
Inner
membrane
Enzyme
complex
1
CoQ
2e–
2e–
Enzyme
complex
2
Enzyme
complex
3
3 ADP + 3 Pi
3 ATP
ATP
synthase
Cyt c
Intermembrane
space
2 H+
2 H+
2 H+
Outer
membrane
26-56
ATP Generated by Oxidation of Glucose
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Glucose
2 ATP
Glycolysis
(net)
2 NADH + 2 H+
Cytosol
2 pyruvate
Mitochondria
2 NADH + 2 H+
CO2
6 NADH + 6 H+
Citric acid
cycle
2 ATP
2 FADH2
Electron-transport
chain
O2
H2O
Figure 26.7
4 ATP
28–30
ATP
Total 36–38
ATP
26-57
Glycogen Metabolism
• ATP is quickly used after it is formed
– It is an energy transfer molecule, not an energy
storage molecule
– Converts the extra glucose to other compounds better
suited for energy storage (glycogen and fat)
• Glycogenesis—synthesis of glycogen
– Stimulated by insulin
– Chains glucose monomers together
26-58
Glycogen Metabolism
• Glycogenolysis—hydrolysis of glycogen
– Releases glucose between meals
– Stimulated by glucagon and epinephrine
– Only liver cells can release glucose back into blood
• Gluconeogenesis—synthesis of glucose from
noncarbohydrates, such as glycerol and amino
acids
– Occurs chiefly in the liver and later, kidneys if necessary
26-59
Major Pathways of Glucose Storage and Use
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Blood
glucose
Extracellular
Intracellular
Glucose
6-phosphatase
(in liver, kidney,
and intestinal cells)
Hexokinase
(in all cells)
Glucose 6-phosphate
Glycogen
synthase
Key
Pi
Glucose
1-phosphate
Glycogenesis
Glycogenolysis
Glycogen
phosphorylase
Glycogen
Pi
Glycolysis
Figure 26.8
26-60
Lipids
• Triglycerides are stored in body’s adipocytes
– Turnover of lipid molecules every 2 to 3 weeks
• Released into blood, transported and either oxidized or
redeposited in other fat cells
• Lipogenesis—synthesis of fat from other types of
molecules
– Amino acids and sugars used to make fatty acids and
glycerol
– PGAL can be converted to glycerol
26-61
Lipids
• Lipolysis—breaking down fat for fuel
– Begins with the hydrolysis of a triglyceride to glycerol
and fatty acids
– Stimulated by epinephrine, norepinephrine,
glucocorticoids, thyroid hormone, and growth hormone
– Glycerol easily converted to PGAL and enters the
pathway of glycolysis
• Generates only half as much ATP as glucose
26-62
Lipids
Cont.
– Beta oxidation in the mitochondrial matrix catabolizes
the fatty acid components
• Removes 2 carbon atoms at a time which bonds to
coenzyme A
• Forms acetyl-CoA, the entry point for the citric acid cycle
– A fatty acid with 16 carbons can yield 129 molecules of
ATP
• Richer source of energy than the glucose molecule
26-63
Lipogenesis and Lipolysis Pathways
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Glucose
Glucose 6-phosphate
Stored
triglycerides
Glycerol
PGAL
Fatty acids
Glycerol
Beta oxidation
Pyruvic
acid
Fatty
acids
Acetyl groups
New
triglycerides
Acetyl-CoA
Ketone bodies
β-hydroxybutyric acid
Acetoacetic acid
Acetone
Citric
acid
cycle
Figure 26.9
Key
Lipogenesis
Lipolysis
26-64
Lipids
• Fatty acids catabolized into acetyl groups (by beta
oxidation in mitochondrial matrix) may:
– Enter citric acid cycle as acetyl-CoA
– Undergo ketogenesis
• Metabolized by liver to produce ketone bodies
– Acetoacetic acid
– -hydroxybutyric acid
– Acetone
• Rapid or incomplete oxidization of fats raises blood ketone
levels (ketosis) and may lead to a pH imbalance
(ketoacidosis)
26-65
Proteins
• Amino acid pool—dietary amino acids plus 100 g
of tissue protein broken down each day into free
amino acids
• May be used to synthesize new proteins
– Fastest rate of cell division is epithelial cells of intestinal
mucosa
• Of all the amino acids absorbed by the small
intestine:
– 50% come from the diet
– 25% from dead epithelial cells
– 25% from enzymes that have digested each other
26-66
Hepatitis and Cirrhosis
• Hepatitis or inflammation of the liver is caused by
one of five strains of hepatitis viruses
– Hepatitis A is common and mild
• Causes up to 6 months of illness, but most recover
– Hepatitis B and C are more serious
• Transmitted sexually and through blood and other body
fluids
• Often lead to cirrhosis or liver cancer
• Cirrhosis—irreversible inflammatory liver disease
– Scar tissue starts to dominate, vessels rupture, and
necrosis occurs
– Results from alcohol abuse
26-67
Metabolic States and Metabolic Rate
• Your metabolism changes from hour to hour
– Depending on how long since your last meal
• Absorptive (fed) state
– About 4 hours during and after a meal
– Nutrients are being absorbed
– Nutrients may be used immediately to meet energy and
other needs
• Postabsorptive (fasting) state
– Prevails in the late morning, late afternoon, and overnight
– Stomach and intestines are empty
– Body’s energy needs are met from stored fuels
26-68
Metabolic Rate
• Metabolic rate—the amount of energy liberated in
the body in a given period of time (kcal/hr or
kcal/day)
– Calorimeter: a closed chamber with water-filled walls
that absorb the heat given off by the body
– Measured indirectly with a spirometer by measuring
the amount of oxygen a person consumes
• Metabolic rate depends on physical activity,
mental state, absorptive or postabsorptive status,
thyroid hormone, and other hormones
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Metabolic Rate
• Basal metabolic rate (BMR)
– A baseline or standard of comparison that minimizes
the effects of the variables
– Relaxed, awake, fasting, comfortable room temperature
– Adult male BMR is 2,000 kcal/day (slightly less for a
female)
26-70
Metabolic Rate
• Total metabolic rate (TMR)—the sum of the BMR
and energy expenditures for voluntary activities,
especially muscle contraction
• Factors raising TMR
– Pregnancy, anxiety, fever, eating, catecholamines and
thyroid hormones
– High in children and decreases with age
• Factors lowering TMR
– Apathy, depression, and prolonged starvation
– As one reduces food intake, the body reduces its
metabolic rate to conserve body mass, making weight
loss more difficult
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Disturbances of Thermoregulation
• Fever
– Normal protective mechanism that should be allowed to
run its course if it is not excessively high
– Above 108° to 110°F can be very dangerous
• Elevates the metabolic rate
• Body generates heat faster than heat-losing mechanisms
can disperse it
• Causes dangerous positive feedback loop
• Core temperatures of 111° to 113°F promote metabolic
dysfunction, neurological damage, and death
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Disturbances of Thermoregulation
• Exposure to excessive heat
– Heat cramps: painful muscle spasms due to electrolyte
imbalance from excessive sweating
• Occur especially when a person begins to relax after
strenuous exertion and heavy sweating
– Heat exhaustion: from severe water and electrolyte
loss
• Hypotension, dizziness, vomiting, and sometimes fainting
26-73
Disturbances of Thermoregulation
Cont.
– Heat stroke (sunstroke): state in which the core body
temperature is over 104°F
• Brought about by prolonged heat wave with high humidity
• Skin is hot and dry
• Nervous system dysfunctions—delirium, convulsions, or
coma
• Tachycardia, hyperventilation, inflammation and
multiorgan dysfunction, death
26-74
Disturbances of Thermoregulation
• Exposure to excessive cold
– Hypothermia can cause life-threatening positive
feedback loops
– If core temperature drops below 91°F
• Metabolic rate drops so low that heat production cannot
keep pace with heat loss
• Temperature falls even more
• Death from cardiac fibrillation may occur below 90°F
• Some people survive body temperatures as low as 84°F
in a state of suspended animation
• Below 74°F is usually fatal
• Dangerous to give alcohol to someone in hypothermia, as
it accelerates heat loss by dilating cutaneous vessels
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Alcohol and Alcoholism
• Alcohol—mind-altering drug, empty calories,
addictive drug, and a toxin
• Rapidly absorbed from GI tract
–
–
–
–
20% in stomach and 80% in small intestine
Carbonation increases rate of absorption
Food reduces absorption
Easily crosses blood–brain barrier to exert intoxicating
effects on the brain
26-76
Alcohol and Alcoholism
• Detoxified by hepatic enzyme alcohol
dehydrogenase which oxidizes it to
acetaldehyde—in citric cycle is oxidized to CO2
and H2O
– Women have less alcohol dehydrogenase and clear
alcohol from the bloodstream more slowly than men
• More vulnerable to alcohol-related illnesses such as
cirrhosis of the liver
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Physiological Effects of Alcohol
• Nervous system
– Depressant because it inhibits the release of
norepinephrine and disrupts GABA receptors
• Low dose provides euphoria and giddiness
• High dose—nerves less responsive to neurotransmitters
– Timing and coordination between neurons is impaired
– Slurred speech, poor coordination, slower reaction time
26-78
Physiological Effects of Alcohol
• Liver
– Excessive acetaldehyde produced during metabolism
causes inflammation of liver and pancreas
• Disruption of digestive function
– Destroys hepatocytes faster than they can regenerate,
producing cirrhosis
• Hepatic coma and jaundice
26-79
Physiological Effects of Alcohol
• Circulatory system
– Clotting problems
• Clotting factors reduced due to liver damage
– Edema due to inadequate production of albumin
– Cirrhosis obstructs hepatic portal circulation
– Portal hypertension and hypoproteinemia
• Liver and other organs “weep” serous fluid into peritoneal
cavity
• Ascites—swelling of abdomen with as much as several
liters of serous fluid
• Hemorrhaging and hematemesis (vomiting blood)
• Destroys myocardial tissue, reduces heart contractility, and
causes arrhythmias
26-80
Physiological Effects of Alcohol
• Digestive system and nutrition
– Breaks down protective mucous barrier of stomach
• Gastritis and bleeding
• Linked to esophageal cancer and peptic ulcers
– Malnutrition from appetite suppression
– Acetaldehyde interferes with vitamin absorption and use
26-81
Addiction
• Alcohol is the most widely available addictive drug
in America
– Similar to barbiturates in toxic effects
• Potential for tolerance, dependence, and risk of
overdose
– Physiological tolerance of high concentrations
– Impaired physiological, psychological, and social
functionality
– Withdrawal symptoms when intake is reduced or
stopped: delirium tremens (DT)
• Restlessness, insomnia, confusion, irritability, tremors,
incoherent speech, hallucinations, convulsions, and coma
• Has 5% to 15% mortality rate
26-82